How Saturn’s Moon Mimas Might Have Melted

You’ve probably heard that Jupiter’s moon Europa and Saturn’s moon Enceladus both hide global oceans beneath their frozen ice crusts. If you’re an outer moons fan, you’ll know that Jupiter’s Ganymede and Saturn’s Titan also have subsurface oceans. But until recently, nobody expected an ocean within Saturn’s small moon Mimas. In a paper just published in Earth and Planetary Science Letters, a group of physicists explain how Mimas could be an ocean world, and that it might be much younger than it looks.

“Usually, we equate oceans with super-active surfaces,” explains Geoff Collins, an outer planets scientist at Wheaton College, who was not involved in the study. “This is why a lot of people (including myself) were incredulous about the recent ocean result.”

Europa, Enceladus, Ganymede, and Titan look very different from Mimas. Their surfaces are smoothed by the effects of subsurface heat, which warms their crust and causes their craters to relax, in the sense of flattening over time. Grooves and ridges cut across their surfaces, degrading or erasing older impact craters.

It’s logical that oceans and activity would be associated; if a moon is warm enough to partially melt, the same heat could drive geology. That’s why it’s so hard to believe that Mimas could have an interior ocean. Its surface is totally saturated with impact craters, including one very large crater, Herschel, whose steep, crisp rim requires that the icy crust has never come close to thawing since the crater first formed.

Two lines of evidence suggest that Mimas actually does have an ocean. Mimas’s rotation and orbital motion are not quite what would be expected if Mimas were a solid body of uniform density. To explain the observations requires Mimas to be denser in the center than at its crust. Two independent groups of researchers concluded that the most plausible model for Mimas’ interior, consistent with the observations, includes an internal ocean beneath about 30 kilometers (20 miles) of solid ice crust.

And yet, its surface looks so old. How to reconcile an old surface with the youthful energy of an internal ocean?

In the paper, Alyssa Rhoden (Southwest Research Institute) and coworkers posit that maybe Mimas is old, but its ocean is young. The shape of Herschel Crater requires that the crust was at least 55 kilometers thick with solid ice when it first formed, or else the impact would have punched through to the ocean beneath, as happened at the Tyre crater on Europa.

“[Rhoden] makes a good point that while the ocean is forming, you don’t necessarily see any evidence on the surface that it’s there,” Collins said in an email. “It’s only after the ocean has gotten large and starts to re-freeze that it begins to wreak havoc on the surface geology.”

A Thinning Crust and an Eccentric Orbit

Even so, if Mimas’s crust used to be more than 55 kilometers thick and is now only 30 kilometers thick, the crust has to be thinning over time. What could cause that?

Intriguingly, the same simulations in which Rhoden and coworkers demonstrate how Mimas’s interior could have melted also show that it may now be done melting. The fact that its orbit is currently eccentric (oval-shape) is somewhat surprising; myriad forces are acting on Mimas to drive its orbit into a more circular state. So the shape of the orbit and the subsurface ocean both require an event not terribly long ago that made its orbit more eccentric. That eccentricity drove tidal heating, which drove melting.

But tides giveth, and they taketh away. Tidal friction dissipates Mimas’ excess eccentricity. As the orbit gets more circular, there’s less tidal friction, and less heat pumped into Mimas’ deep interior. Rhoden’s simulations show that Mimas may have reached or recently passed a tipping point and is now on its way to being frozen solid.

There’s other evidence for recent change in the Saturn system. Cassini researchers have concluded that the ring system is young, maybe 100 million years old. The Cassini Division in the rings is younger yet, and its very existence suggests that Mimas’ orbit has been shifting, because it’s Mimas’s gravitational tugs that clear out that ring gap.

It's even possible that Mimas is as young as, or even younger than, the rings. A moon as small as Mimas, orbiting as close as it does to Saturn, should not have survived the age of the solar system without being destroyed in a high-speed collision. Another of Saturn’s moons may have disintegrated recently to create the ring system we see today. It’s possible that Mimas was actually born of the death of this previous moon, emerging from the youthful ring system and coalescing from the outermost portion of debris. In that case, its heavily cratered surface would speak to the violent environment of the neighborhood of Saturn’s rings right after they formed, not a solar system age’s worth of impacts.

“The thing is you need to have a place that has been dormant for a long, long time, and then catch it in the act of first starting to wake up,” Collins says. It doesn’t seem very likely that we’d be lucky enough to be observing in the window of time of a few tens of millions of years when a moon is waking up. But if you visit enough moons, sooner or later you’ll catch one in the act — and that might be the case with Mimas.